The present invention relates to steering control for a spacecraft, and more particularly to a control arrangement capable of converting arbitrary spacecraft boresight commands into torque and gimbal rate commands for a pair of spinning control moment gyros (CMG).
A CMG is a spinning momentum wheel that is gimbaled with respect to a spacecraft body about one or both axes nominally orthogonal to its momentum vector, thus permitting controlled rotation of the wheel momentum vector in the spacecraft body. Such rotation allows very large torques to be applied to the spacecraft body, thereby producing rapid acceleration. CMGs are typically employed when a large spacecraft has to be reoriented rapidly and frequently such that fuel consumption associated with using thrusters is impractical.
In most agile spacecraft applications, high torque agile motion is only required about two axes, i.e., roll and pitch, in order to point some a payload line-of-sight toward a target. Rotation of a single CMG produces a rotating torque in the spacecraft body. Typically three active CMGs are employed with some advantageous mounting geometry (e.g., gimbal axes on the faces of a pyramid) to produce a 3-axis torque for body control.
Many control laws and multiple CMG array configurations have been proposed to satisfy required 3-axis torque and momentum storage requirements. One of the more useful is known as a scissor pair array. The array employs two opposing wheel momentum vectors using anti-parallel gimbal axes. The control law constrains the gimbal rates and angles to be nominally identical. This results in a single axis body fixed torque that can be oriented along any desired axis by properly orienting the wheels. However, should the need arise to realize frequent rapid reorientations of a large spacecraft, six wheels would be required using a standard scissor pair on each of 3 axes to achieve independent uncoupled orthogonal three-axis control, while additional wheels would be required for standby redundancy in the event of failure. Thus, such an implementation is undesirably bulky, expensive, and heavy, all being crucial factors for spacecraft applications.
As an improvement to such a scissor pair implementation, commonly owned U.S. Pat. No. 5,681,012 to Rosmann et al. discloses a spacecraft control arrangement having a skewed scissor pair (SSP) CMG configuration and associated steering law. The SSP CMG steering arrangement provides a simple, lighter weight, and less expensive way to control spacecraft attitude with two axis agility and high reliability. While such an arrangement provides improved spacecraft steering control, a need still exists for a steering control process for a SSP actuated spacecraft which provides precise boresight control while also accommodating some error in xe2x80x9cyawxe2x80x9d about the boresight trajectory.
It is therefore an object of the present invention to provide a method and system for steering a SSP actuated spacecraft which satisfies the above-noted need to provide tight boresight control while allowing a certain amount of error in yaw about the desired boresight.
It is another object of the present invention to provide a steering control arrangement for a spacecraft which allows the body of the spacecraft having a body-fixed payload boresight to be steered over arbitrary trajectories using a skewed scissor pair CMG set.
In accordance with these and other objects, the present invention provides a steering control process for converting arbitrary spacecraft boresight commands into gimbal torque or gimbal rate commands for use by a pair of CMGs in a skewed scissor pair (SSP) configuration. The steering control process is implemented in a computer based control system on board a spacecraft carrying such SSP CMGs.
In accordance with one aspect of the present invention, a method is provided for steering a spacecraft along a desired boresight trajectory, where the spacecraft includes a skewed scissor pair control moment gyro (SSP CMG) attitude control arrangement gimbaled with respect to the spacecraft body. The method includes determining trajectory accelerations corresponding to the desired boresight trajectory, determining corresponding constraints on three-axis body accelerations as a function of the desired two-axis boresight accelerations, determining body control torques consistent with a two-dimensional torque authority from SSP CMG gimballing, and determining SSP CMG gimbal rates as a function of these body control torques. The gimbal rates are used to generate torque control signals for controlling operation of SSP CMG to steer the spacecraft along the desired boresight.
In addition, in order to limit boresight yaw perturbations introduced by the torquing of the present invention, a degree of freedom control can be added by varying the toque command to a low authority torque generating device such as the CMG wheel motors. A determination is made whether the existing torque capacity of the wheel spin motor torque drive elements associated with each CMG is exceeded if torques sufficient to follow the boresight trajectory and null yaw errors are commanded. If the spin motor torque capacity is exceeded, their commands are saturated to bound yaw perturbations, and new gimbal rates are determined based on retaining precise boresight trajectory following under this perturbation.
In accordance with another aspect of the present invention, a method for steering a spacecraft having a skewed scissor pair control moment gyro (SSP CMG) attitude control arrangement gimbaled with respect to the spacecraft body includes selecting a desired boresight trajectory which is fixed relative to the spacecraft body, determining boresight angular accelerations based on the desired boresight trajectory, and translating the determined boresight angular accelerations into a set of corresponding SSP CMG gimbal rates. A determination is then made regarding whether an existing torque capacity of a motor torque drive element associated with each SSP CMG is exceed by the determined gimbal rates. If the existing torque capacity is exceeded, new gimbal rates are determined based on a reduced boresight acceleration.
In accordance with yet another aspect of the present invention, a method is provided for controlling a spacecraft attitude that includes providing at least two torque generation devices, and orienting the torque generation devices so that their torque generation vectors are not collinear. A selected boresight direction fixed relative to the spacecraft body, and the spacecraft attitude is measured using spacecraft sensors and forming corresponding pitch, roll, and yaw signals about the boresight. A trajectory command is accepted for boresight pitch and roll, and torque commands are generated based on the signals for pitch, roll and yaw about the boresight and the accepted trajectory command for boresight pitch and roll. The generated torque commands are applied through an actuator law to the at least two torque generation devices.